Period-luminosity relation

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Period-Luminosity relation for Classical Cepheid variables. Storm2011 Cepheid Data.svg
Period-Luminosity relation for Classical Cepheid variables.

In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, sometimes called the Leavitt Law. [2] [3] [4] Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances. [5] [6] [7] [8] [9] [10] The physical model explaining the Leavitt's law for classical cepheids is called kappa mechanism .

Contents

History

Plot from Leavitt's 1912 paper. The horizontal axis is the logarithm of the period of the corresponding Cepheid, and the vertical axis is its apparent magnitude. The lines drawn correspond to the stars' minimum and maximum brightness. HSLeavittHSCr13Fig2 1912.jpg
Plot from Leavitt's 1912 paper. The horizontal axis is the logarithm of the period of the corresponding Cepheid, and the vertical axis is its apparent magnitude. The lines drawn correspond to the stars' minimum and maximum brightness.

Leavitt, a graduate of Radcliffe College, worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. Observatory Director Edward Charles Pickering assigned Leavitt to the study of variable stars of the Small and Large Magellanic Clouds, as recorded on photographic plates taken with the Bruce Astrograph of the Boyden Station of the Harvard Observatory in Arequipa, Peru. She identified 1777 variable stars, of which she classified 47 as Cepheids. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period. [13] Building on this work, Leavitt looked carefully at the relation between the periods and the brightness of a sample of 25 of the Cepheids variables in the Small Magellanic Cloud, published in 1912. [11] This paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt".

In the 1912 paper, Leavitt graphed the stellar magnitude versus the logarithm of the period and determined that, in her own words,

A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods. [11]

Using the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately the same distance, the apparent magnitude of each star is equivalent to its absolute magnitude offset by a fixed quantity depending on that distance. This reasoning allowed Leavitt to establish that the logarithm of the period is linearly related to the logarithm of the star's average intrinsic optical luminosity (which is the amount of power radiated by the star in the visible spectrum). [14]

At the time, there was an unknown scale factor in this brightness since the distances to the Magellanic Clouds were unknown. Leavitt expressed the hope that parallaxes to some Cepheids would be measured; one year after she reported her results, Ejnar Hertzsprung determined the distances of several Cepheids in the Milky Way and that, with this calibration, the distance to any Cepheid could then be determined. [14]

The relation was used by Harlow Shapley in 1918 to investigate the distances of globular clusters and the absolute magnitudes of the cluster variables found in them. It was hardly noted at the time that there was a discrepancy in the relations found for several types of pulsating variable all known generally as Cepheids. This discrepancy was confirmed by Edwin Hubble's 1931 study of the globular clusters around the Andromeda Galaxy. The solution was not found until the 1950s, when it was shown that population II Cepheids were systematically fainter than population I Cepheids. The cluster variables (RR Lyrae variables) were fainter still. [15]

The relations

Period-luminosity relations are known for several types of pulsating variable stars: type I Cepheids; type II Cepheids; RR Lyrae variables; Mira variables; and other long-period variable stars. [16]

Classical Cepheids

Period-Luminosity Relation for Cepheids Period-Luminosity Relation for Cepheids.png
Period-Luminosity Relation for Cepheids

The Classical Cepheid period-luminosity relation has been calibrated by many astronomers throughout the twentieth century, beginning with Hertzsprung. [17] Calibrating the period-luminosity relation has been problematic; however, a firm Galactic calibration was established by Benedict et al. 2007 using precise HST parallaxes for 10 nearby classical Cepheids. [18] Also, in 2008, ESO astronomers estimated with a precision within 1% the distance to the Cepheid RS Puppis, using light echos from a nebula in which it is embedded. [19] However, that latter finding has been actively debated in the literature. [20]

The following relationship between a Population I Cepheid's period P and its mean absolute magnitude Mv was established from Hubble Space Telescope trigonometric parallaxes for 10 nearby Cepheids:

with P measured in days. [21] [18] The following relations can also be used to calculate the distance to classical Cepheids.

Impact

Phase lightcurve of variable star Delta Cephei. Delta Cephei lightcurve.jpg
Phase lightcurve of variable star Delta Cephei.

Classical Cepheids (also known as Population I Cepheids, type I Cepheids, or Delta Cepheid variables) undergo pulsations with very regular periods on the order of days to months. Cepheid variables were discovered in 1784 by Edward Pigott, first with the variability of Eta Aquilae, [22] and a few months later by John Goodricke with the variability of Delta Cephei, the eponymous star for classical Cepheids. [23] Most of the Cepheids were identified by the distinctive light curve shape with a rapid increase in brightness and a sharp turnover.

Classical Cepheids are 4–20 times more massive than the Sun [24] and up to 100,000 times more luminous. [25] These Cepheids are yellow bright giants and supergiants of spectral class F6 – K2 and their radii change by of the order of 10% during a pulsation cycle. [26]

Leavitt's work on Cepheids in the Magellanic Clouds led her to discover the relation between the luminosity and the period of Cepheid variables. Her discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies. Cepheids were soon detected in other galaxies, such as Andromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are independent galaxies located far outside of the Milky Way. Leavitt's discovery provided the basis for a fundamental shift in cosmology, as it prompted Harlow Shapley to move the Sun from the center of the galaxy in the "Great Debate" and Hubble to move the Milky Way galaxy from the center of the universe. With the period-luminosity relation providing a way to accurately measure distances on an inter-galactic scale, a new era in modern astronomy unfolded with an understanding of the structure and scale of the universe. [27] The discovery of the expanding universe by Georges Lemaitre and Hubble were made possible by Leavitt's groundbreaking research. Hubble often said that Leavitt deserved the Nobel Prize for her work, [28] and indeed she was nominated by a member of the Swedish Academy of Sciences in 1924, although as she had died of cancer three years earlier she was not eligible. [29] [30] (The Nobel Prize is not awarded posthumously.)

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<span class="mw-page-title-main">Large Magellanic Cloud</span> Satellite galaxy of the Milky Way

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<span class="mw-page-title-main">Variable star</span> Star whose brightness fluctuates, as seen from Earth

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<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

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<span class="mw-page-title-main">Magellanic Clouds</span> Two dwarf galaxies orbiting the Milky Way

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<span class="mw-page-title-main">Small Magellanic Cloud</span> Dwarf irregular galaxy, satellite galaxy of the Milky Way

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<span class="mw-page-title-main">Henrietta Swan Leavitt</span> American astronomer (1868–1921)

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<span class="mw-page-title-main">Delta Cephei</span> Binary star system in the constellation Cepheus

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<span class="mw-page-title-main">W Sagittarii</span> Star in the constellation Sagittarius

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<span class="mw-page-title-main">Type II Cepheid</span>

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<span class="mw-page-title-main">Classical Cepheid variable</span>

Classical Cepheids are a type of Cepheid variable star. They are young, population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes ranging from a few tenths of a magnitude up to about 2 magnitudes. Classical Cepheids are also known as Population I Cepheids, Type I Cepheids, and Delta Cepheid variables.

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Michael William Feast was a British-South African astronomer. He served as Director of the South African Astronomical Observatory from 1976–1992, then became a professor at the University of Cape Town.

<span class="mw-page-title-main">S Vulpeculae</span> Variable star in the constellation Vulpecula

S Vulpeculae is a variable star located in the constellation Vulpecula. A supergiant star, it is around 382 times the diameter of the Sun.

<span class="mw-page-title-main">NGC 121</span> Globular cluster in the constellation Tucana

NGC 121 is a globular cluster of stars in the southern constellation of Tucana. It is the oldest globular cluster in the Small Magellanic Cloud (SMC), which is a dwarf satellite galaxy of the Milky Way. This cluster was first discovered by English astronomer John Herschel on September 20, 1835. The compiler of the New General Catalogue, Danish astronomer John Louis Emil Dreyer, described this object as "pretty bright, pretty small, little extended, very gradually brighter middle". The cluster is located at a distance of around 200,000 light-years (60 kpc) from the Sun.

<span class="mw-page-title-main">X Cygni</span> Variable star in the constellation Cygnus

X Cygni is a variable star in the northern constellation of Cygnus, abbreviated X Cyg. This is a Delta Cephei variable that ranges in brightness from an apparent visual magnitude of 5.85 down to 6.91 with a period of 16.386332 days. At it brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 628 light years based on parallax measurements. It is drifting further away with a radial velocity of 8.1 km/s. This star is a likely member of the open cluster Ruprecht 173.

References

  1. ↑ Storm, J.; Gieren, W.; Fouqué, P.; Barnes, T. G.; Pietrzyński, G.; Nardetto, N.; Weber, M.; Granzer, T.; Strassmeier, K. G. (October 2011). "Calibrating the Cepheid period-luminosity relation from the infrared surface brightness technique: I. The p-factor, the Milky Way relations, and a universal K-band relation". Astronomy & Astrophysics. 534. EDP Sciences: A94. arXiv: 1109.2017 . Bibcode:2011A&A...534A..94S. doi:10.1051/0004-6361/201117155. S2CID   96456751.
  2. ↑ Johnson, Kirk (March 27, 2024). "Overlooked No More: Henrietta Leavitt, Who Unraveled Mysteries of the Stars - The portrait that emerged from her discovery, called Leavitt's Law, showed that the universe was hundreds of times bigger than astronomers had imagined". The New York Times . Archived from the original on March 27, 2024. Retrieved March 28, 2024.
  3. ↑ "A century of cepheids: Two astronomers, a hundred years apart, use stars to measure the Universe" (Press release). Sloan Digital Sky Survey. 9 January 2018. Retrieved 23 September 2019.
  4. ↑ "Society Resolutions". American Astronomical Society. 8 March 2021. Archived from the original on 8 March 2021.
  5. ↑ Udalski, A.; Soszynski, I.; Szymanski, M.; Kubiak, M.; Pietrzynski, G.; Wozniak, P.; Zebrun, K. (1999). "The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud". Acta Astronomica. 49: 223–317. arXiv: astro-ph/9908317 . Bibcode:1999AcA....49..223U.
  6. ↑ Soszynski, I.; Poleski, R.; Udalski, A.; Szymanski, M. K.; Kubiak, M.; Pietrzynski, G.; Wyrzykowski, L.; Szewczyk, O.; Ulaczyk, K. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud". Acta Astronomica. 58: 163. arXiv: 0808.2210 . Bibcode:2008AcA....58..163S.
  7. ↑ Freedman, Wendy L.; Madore, Barry F.; Gibson, Brad K.; Ferrarese, Laura; Kelson, Daniel D.; Sakai, Shoko; Mould, Jeremy R.; Kennicutt, Jr., Robert C.; Ford, Holland C.; Graham, John A.; Huchra, John P.; Hughes, Shaun M. G.; Illingworth, Garth D.; Macri, Lucas M.; Stetson, Peter B. (2001). "Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant". The Astrophysical Journal. 553 (1): 47–72. arXiv: astro-ph/0012376 . Bibcode:2001ApJ...553...47F. doi:10.1086/320638. S2CID   119097691.
  8. ↑ Tammann, G. A.; Sandage, A.; Reindl, B. (2008). "The expansion field: the value of H 0". The Astronomy and Astrophysics Review. 15 (4): 289–331. arXiv: 0806.3018 . Bibcode:2008A&ARv..15..289T. doi:10.1007/s00159-008-0012-y. S2CID   18463474.
  9. ↑ Majaess, D. J.; Turner, D. G.; Lane, D. J. (2009). "Characteristics of the Galaxy according to Cepheids". Monthly Notices of the Royal Astronomical Society. 398 (1): 263–270. arXiv: 0903.4206 . Bibcode:2009MNRAS.398..263M. doi:10.1111/j.1365-2966.2009.15096.x. S2CID   14316644.
  10. ↑ Freedman, Wendy L.; Madore, Barry F. (2010). "The Hubble Constant". Annual Review of Astronomy and Astrophysics. 48: 673–710. arXiv: 1004.1856 . Bibcode:2010ARA&A..48..673F. doi:10.1146/annurev-astro-082708-101829. S2CID   119263173.
  11. 1 2 3 Leavitt, Henrietta S.; Pickering, Edward C. (1912). "Periods of 25 Variable Stars in the Small Magellanic Cloud". Harvard College Observatory Circular. 173: 1–3. Bibcode:1912HarCi.173....1L.
  12. ↑ Kerri Malatesta (16 July 2010). "Delta Cephei". American Association of Variable Star Observers.
  13. ↑ Leavitt, Henrietta S. (1908). "1777 variables in the Magellanic Clouds". Annals of Harvard College Observatory. 60: 87–108. Bibcode:1908AnHar..60...87L.
  14. 1 2 Fernie, J.D. (December 1969). "The Period–Luminosity Relation: A Historical Review". Publications of the Astronomical Society of the Pacific. 81 (483): 707. Bibcode:1969PASP...81..707F. doi: 10.1086/128847 .
  15. ↑ Baade, W. (1956). "The Period-Luminosity Relation of the Cepheids". Publications of the Astronomical Society of the Pacific. 68 (400): 5. Bibcode:1956PASP...68....5B. doi: 10.1086/126870 .
  16. ↑ Sesar, Branimir; Fouesneau, Morgan; Price-Whelan, Adrian M.; Bailer-Jones, Coryn A. L.; Gould, Andy; Rix, Hans-Walter (2017). "A Probabilistic Approach to Fitting Period–luminosity Relations and ValidatingGaia Parallaxes". The Astrophysical Journal. 838 (2): 107. arXiv: 1611.07035 . Bibcode:2017ApJ...838..107S. doi: 10.3847/1538-4357/aa643b .
  17. ↑ Hertzsprung, Ejnar (1913). "Über die räumliche Verteilung der Veränderlichen vom δ Cephei-Typus". Astronomische Nachrichten. 196: 201. Bibcode:1913AN....196..201H.
  18. 1 2 Benedict, G. Fritz; McArthur, Barbara E.; Feast, Michael W.; Barnes, Thomas G.; Harrison, Thomas E.; Patterson, Richard J.; Menzies, John W.; Bean, Jacob L.; Freedman, Wendy L. (2007). "Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations". The Astronomical Journal. 133 (4): 1810. arXiv: astro-ph/0612465 . Bibcode:2007AJ....133.1810B. doi:10.1086/511980.
  19. ↑ Kervella, P.; Mérand, A.; Szabados, L.; Fouqué, P.; Bersier, D.; Pompei, E.; Perrin, G. (2008). "The long-period Galactic Cepheid RS Puppis". Astronomy and Astrophysics. 480 (1): 167. arXiv: 0802.1501 . Bibcode:2008A&A...480..167K. doi:10.1051/0004-6361:20078961. S2CID   14865683.
  20. ↑ Bond, H. E.; Sparks, W. B. (2009). "On geometric distance determination to the Cepheid RS Puppis from its light echoes". Astronomy and Astrophysics. 495 (2): 371. arXiv: 0811.2943 . Bibcode:2009A&A...495..371B. doi:10.1051/0004-6361:200810280.
  21. ↑ Benedict, G. Fritz; McArthur, B. E.; Fredrick, L. W.; Harrison, T. E.; Slesnick, C. L.; Rhee, J.; Patterson, R. J.; Skrutskie, M. F.; Franz, O. G.; Wasserman, L. H.; Jefferys, W. H.; Nelan, E.; Van Altena, W.; Shelus, P. J.; Hemenway, P. D.; Duncombe, R. L.; Story, D.; Whipple, A. L.; Bradley, A. J. (2002). "Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator δ Cephei". The Astronomical Journal. 124 (3): 1695. arXiv: astro-ph/0206214 . Bibcode:2002AJ....124.1695B. doi:10.1086/342014. S2CID   42655824.
  22. ↑ Pigott, Edward (1785). "Observations of a new variable star". Philosophical Transactions of the Royal Society. 75: 127–136. Bibcode:1785RSPT...75..127P. doi:10.1098/rstl.1785.0007. S2CID   186212958.
  23. ↑ Goodricke, John (1786). "A series of observations on, and a discovery of, the period of the variation of the light of the star marked δ by Bayer, near the head of Cepheus. In a letter from John Goodricke, Esq. to Nevil Maskelyne, D.D.F.R.S. and Astronomer Royal". Philosophical Transactions of the Royal Society of London. 76: 48–61. Bibcode:1786RSPT...76...48G. doi: 10.1098/rstl.1786.0002 .
  24. ↑ Turner, David G. (1996). "The Progenitors of Classical Cepheid Variables". Journal of the Royal Astronomical Society of Canada. 90: 82. Bibcode:1996JRASC..90...82T.
  25. ↑ Turner, David G. (2010). "The PL calibration for Milky Way Cepheids and its implications for the distance scale". Astrophysics and Space Science. 326 (2): 219–231. arXiv: 0912.4864 . Bibcode:2010Ap&SS.326..219T. doi:10.1007/s10509-009-0258-5. S2CID   119264970.
  26. ↑ Rodgers, A. W. (1957). "Radius variation and population type of cepheid variables". Monthly Notices of the Royal Astronomical Society . 117: 85–94. Bibcode:1957MNRAS.117...85R. doi: 10.1093/mnras/117.1.85 .
  27. ↑ "1912: Henrietta Leavitt Discovers the Distance Key." Everyday Cosmology. N.p., n.d. Web. 20 Oct. 2014. "1912: Henrietta Leavitt Discovers the Distance Key | Everyday Cosmology". Archived from the original on 2014-06-04. Retrieved 2016-10-05.
  28. ↑ Ventrudo, Brian (November 19, 2009). "Mile Markers to the Galaxies". One-Minute Astronomer. Archived from the original on March 12, 2015. Retrieved September 24, 2019.
  29. ↑ Singh, Simon (2005). Big Bang: The Origin of the Universe. Harper Perennial. Bibcode:2004biba.book.....S. ISBN   978-0-00-715252-0.
  30. ↑ Johnson, George (2005). Miss Leavitt's Stars : The Untold Story of the Woman Who Discovered How To Measure the Universe (1st ed.). New York: Norton. ISBN   978-0-393-05128-5.
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